[umeshunni]

In my experience (US, SF Bay Area/Seattle) - people do this for a few reasons:

1. The coordination and time overhead of sharing their commute with another person is not worth the marginal benefits - HOV lanes are not present in most of the area to make a difference in commute time and gas prices are low enough that splitting a drive isn't worth the savings.

2. Their commute also involves other errands (shopping, going to the gym, dropping off or picking up children from school/daycare) that are inconvenient to do with another person in the car.

In summary, it's convenient to drive alone and you are not penalized sufficiently for doing so.